Referring to FIG. 12, an example of a high-frequency BPF (Band Pass Filter) will be described.
As shown in FIG. 12, this high-frequency BPF is formed by dividing a rectangular waveguide into two halves 110 and 120 along a signal propagation direction at the center of an H-plane, and interposing a thin metal fin 130 having a plurality of windows between these two halves 110 and 120. This type of high-frequency BPF is also referred to as an E-plane waveguide-type BPF.
The characteristics of the E-plane waveguide-type BPF are determined depending on the shapes of the metal fin 130 and the waveguide (particularly, the length of the long side (width) of the cross-section of the rectangular waveguide). Therefore, the shape of the metal fin 130 or the cross-sectional shape of the rectangular waveguide must be changed in order to change, for example, the central frequency of the BPF.
Japanese Laid-Open Patent Publication No. 2007-88545 (Patent Document 1) discloses a BPF which is designed such that the central frequency or frequency bandwidth can be electrically adjusted in order to enlarge the coverable frequency bandwidth.
Briefly describing this BPF, the metal fin 130 shown in FIG. 12 is replaced with a three-layer substrate having a resonator, and the resonator has an active element provided therein. In this BPF, the central frequency or bandwidth is adjusted by applying a bias voltage from the outside of the three-layer substrate to the active element through a line pattern in the intermediate layer of the three-layer substrate.